Ovigeny index increases with temperature in an aphid parasitoid: Is early reproduction better when it is hot?

Studying relative investment of resources towards early and delayed reproduction is central to understand life history evolution since these traits are generally negatively correlated and traded-off against several other fitness components. For this purpose, ovigeny index (OI), which is calculated as the fraction of the maximum potential lifetime egg complement that is mature upon female emergence, has been developed in insects. Despite the central role of temperature on life history evolution in ectotherms, its influence on ovigeny index has never been tested. Adaptive models imply that OI should increase with temperature because of changes in body size, but the same influence may be expected considering physiological effects of temperature on egg maturation rate or amount of energy available. We investigated in the aphid parasitoid Aphidius ervi the influence of temperature experienced by the immature and/or the adult (from 12 °C to 28 °C) on ovigeny index and oviposition behaviour. As predicted, OI increased between 16 and 28 °C, i.e. females were able to reproduce earlier as temperature increased but this was traded off against a lower delayed reproduction. The highest OI was however observed at 12°, probably because this temperature was too low for females to mature eggs. Females that developed at 20 °C and were transferred as adult at 24 °C and 28 °C had the highest ovigeny index and laid more eggs during the early oviposition period while those transferred at 16 °C laid more eggs at the end of their life. Our results suggest that ovigeny index is not only influenced by body size - i.e. the adaptive explanation - but also by adult egg maturation rate, lifespan or amount of energy available - i.e. a physiological and adaptive explanation.